1. Field of the Invention
This invention relates in general to power supply systems. Specifically, the invention provides a power supply system which instantaneously switches an inverter to another reserved power source so as to supply power to a load without any interruption. This invention also provides a method for controlling a power supply system.
2. Description of the Prior Art
Hitherto there has been widely used a power supply system provided with changeover switches to switch a load from an output of an inverter to a stand-by commercial power source (a reserved power source). In normal operation, the inverter's output supplies power to the load through the changeover switches. If the inverter fails or must be shut down for maintenance, the system causes the changeover switches to switch the load from the inverter's output to the stand-by commercial power source so as to directly supply power to the load. When the inverter is again operational, the system causes the changeover switches to switch the load back to the inverter's output so as to continuously supply power to the load. As an example of such a conventional power supply system, "Uninterruptible Power Supplies with Inverters" are described in the Siemens Review XL (1973) No. 8, pages 365-368.
In the above-described conventional power supply system, should a load current exceed the current capacity of the inverter when the inverter is supplying power to the load, the inverter is tripped by the output of a protective device provided in the power supply system. At the same time the changeover switches are caused, as in the above-described case of inverter failure, to switch the inverter to the stand-by commercial power source which then provides power to the load. The conventional power supply system, described above, has some operational difficulties and disadvantages.
The inverter usually trips upon start-up operation of a load such as a motor or a transformer through which an excessive current flows at its start-up operation. Thus, at every start-up of the load, the inverter trips and must be restarted so that the load can then be switched back to the inverter.
In the conventional power supply system, a current limiting operation may be employed. With current limiting, there is a rapid reduction of output voltage of the inverter and tripping is avoided. However, with current limiting operation, the load "sees" an instantaneous interruption of power. In addition, in the case of a short-circuit failure on the load side, it is difficult for the inverter to supply a sufficient short-circuit current so as to perform a selective interruption. As a result, it is difficult to distinguish a short-circuited feeder from a normal feeder.
One proposal to address the above-described problems was to make the changeover switch a semiconductor interrupter provided with a forced commutation circuit in the power supply system. However, such a power supply system is expensive to manufacture because a semiconductor interrupter is an expensive component. Furthermore, the reliability of such a system becomes low because of the increased number of components required in such a power supply system.